In recent years, positive electrodes for alkaline storage batteries have been improved to have an increasingly improved capacity density by means of the improvements of substrate form, type of active material, composition of active material and additives. Positive electrodes having a capacity density of about 600 mAh/cc are currently in practical use. However, devices which use alkaline storage batteries as their power sources require for the batteries to have more improved high-rate discharge characteristics and more enhanced output. In order to improve the high-rate discharge characteristics, the methods to improve the current-collecting efficiency of the electrode, to reduce the resistance of the electrode and to enhance the charge/discharge efficiency of the active material have been investigated. The method to replace nickel contained in nickel hydroxide with other metals so as to reform nickel hydroxide has also been examined.
Since a solid solution nickel hydroxide containing a small amount of magnesium has high discharge potential, its application to an electrode material has been examined. If the discharge potential is shifted to noble direction, the battery output will be increasingly improved. The use of a solid solution nickel hydroxide containing a small amount of magnesium as the positive electrode active material improves the battery's cycle life because the production of γ-NiOOH is suppressed.
The following are proposed examples of a solid solution nickel hydroxide containing a small amount of magnesium.
(1) Japanese Laid-Open Patent Publication No. Hei 2-109261 proposes a solid solution nickel hydroxide containing 1 to 3 wt % of magnesium, in which the total volume of micropores with a diameter of 30 Å or less is 0.05 ml/g or less, to be used as the positive electrode active material. This prior art example is intended to obtain high-density nickel hydroxide powder, to prevent the production of γ-NiOOH by the addition of magnesium, and to improve the active material utilization rate.
(2) Japanese Laid-Open Patent Publication No. Hei 5-21064 proposes a mixture of spherical or sphere-like particles and non-spherical particles to be used as the positive electrode active material, in which 1 to 7 wt % of magnesium or the like of the amount of nickel hydroxide powders is added during the production of the positive electrode. This prior art example is intended to improve the packing density of nickel hydroxide when it is filled into the positive electrode, to prevent the production of γ-NiOOH during overcharge by adding magnesium or the like, and to improve the battery's cycle life.
(3) Japanese Laid-Open Patent Publication No. Hei 5-41212 proposes a nickel hydroxide containing 1 to 7 wt % of magnesium or the like of the amount of nickel hydroxide powder to be used as the positive electrode active material, in which innumerable primary particles with a particle size of 0.1 μm or less are aggregated and the volume of a micropore with a diameter of 30 Å or more is 20 to 70% of the total volume of the micropores. This prior art example is intended to prevent the production of γ-NiOOH due to the uneven distribution of the electrolyte in the particles by facilitating the impregnation of the electrolyte into the particles, and to improve the active material utilization rate at the initial charge/discharge cycle. The aim of adding magnesium or the like is to improve the battery's cycle life, which is the same as that of the prior art example (2).
(4) Japanese Laid-Open Patent Publication No. Hei 5-182662 proposes a solid solution nickel hydroxide containing an additive element to be used as the positive electrode active material in which the volume of internal micropore is 0.14 ml/g or less. Examples of the additive element are Zn, Mg, Cd or Ba, all of which do not degrade the characteristics of the nickel hydroxide active material. This prior art example is intended to suppress the production of γ-NiOOH by forming defects in the crystal lattice of nickel hydroxide as well as increasing the mobility of protons in the high-density nickel hydroxide powders in which the volume of the internal micropore is small, and to improve the battery's cycle life.
(5) Japanese Laid-Open Patent Publication No. Hei 5-182663 proposes a solid solution nickel hydroxide containing Co and other additive elements, in which the volume of an internal micropore is 0.14 ml/g, to be used as the positive electrode active material. Examples of the additive element are Zn, Mg, Cd or Ba. This prior art example is intended to improve the high temperature charge efficiency, to suppress the production of γ-NiOOH, and to enhance the battery's cycle life.
(6) Japanese Laid-Open Patent Publication No. Hei 11-219703 proposes, as the positive electrode active material, composite particles comprising: solid solution nickel hydroxide particles containing 0.5 wt % to 5 wt % of magnesium of the amount of nickel; and a coating layer containing sodium and cobalt compound formed on the solid solution nickel hydroxide particles. It is also proposed that yttrium metal and/or yttrium compound in an amount of 0.05 to 5.0 wt %, in yttrium percentage, of the nickel amount in the nickel hydroxide particle is/are added to the positive electrode. This prior art example is intended to suppress the production of γ-NiOOH, to improve the battery's cycle life, and to enhance the charge efficiency by providing a coating layer containing sodium and cobalt compound, and yttrium.
Therefore, all of the prior art examples (1) through (6) are intended to improve the charge/discharge efficiency and the battery's cycle life.
The problem arises, however, that the utilization rate of a solid solution nickel hydroxide containing magnesium tends to decrease during high rate discharge. This is because, when the solid solution nickel hydroxide is synthesized, the growth of nickel hydroxide crystals is hindered, thereby crystal grains become smaller to increase the specific surface area. If the specific surface area is increased, the electrolyte will be excessively absorbed into the positive electrode to reduce the amount of electrolyte in the separator or the negative electrode. Due to the increased specific surface area of nickel hydroxide, the coating layer composed of cobalt oxide grows unevenly to make the crystals of cobalt oxide nonuniform, resulting in reduction of the conductivity of the active material. As a result, the polarization during high rate discharge becomes larger and the active material utilization rate is decreased.
Another problem arises that a solid solution nickel hydroxide containing magnesium has a significantly reduced charge efficiency in high temperatures. This is because the solid solution nickel hydroxide containing magnesium has not only an increased discharge potential but also an increased charge potential; therefore, the oxygen evolution reaction is likely to proceed at the end of charging.
Accordingly, even if batteries are produced according to the prior art examples (1) to (6), they will not have satisfactory high rate charge/discharge characteristics and will not excel in high temperature charge efficiency. According to the prior art example (5), charge efficiency will be slightly improved because the charge potential is reduced due to the lowering of the potential of the electrode comprising nickel hydroxide containing cobalt. However, the effect of improving charge efficiency is poor, and when a solid solution nickel hydroxide containing magnesium with a high charge potential is used as the positive electrode active material, it is impossible to raise the charge efficiency to the required level. According to the prior art example (6), the charge efficiency will be improved because the addition of yttrium metal or yttrium compound to the positive electrode increases the oxygen generating overvoltage at the end of charging. When the solid solution nickel hydroxide contains a large amount of magnesium, however, the charge potential will be significantly increased; therefore, it is difficult to achieve a required high level of charge efficiency.
In addition to the above, the prior art examples (1) to (6) are not intended to produce a battery of a greater output by utilizing a high discharge potential of a solid solution nickel hydroxide containing magnesium.